Magnetron oscillator



April 1951 M. J. o. STRUTT ET AL 2,548,567

' MAGNETRON OSCILLATOR Filed Sept. 18, 1946 I INVENTORS MAXIMILIAAN JULIUS OTTO sTRurT xonusus SWIER KNOL 410mm: DER ZIEL.

- BY l 7 AT ORNET.

Patented Apr. 10, 1951 UNITED STATES PATENT OFFICE MAGNETRON OSCILLATOR trustee Application September 18, 1946, Serial No. 697,770 In the Netherlands September 3, 1943 Section 1, Public Law 690, August 8, 194.6

Patent expires September 3, 1963 7 Claims. 1

The copending application S. N. 682,983, filed July 12, 1946, which issued on December 19, 1950, as Patent Number 2,534,836, relates to a circuit for the transmission or production of electrical oscillations of very high frequency, more particularly of oscillations of the Ho-type, with the aid of a discharge tube whose electrode system has the form of one or more hollow waveguides the dimension of which in a direction normal to the direction of propagation of the oscillations in the hollow wave-guides and normal to the dir ction of the electronic current in the tube is larger than half of the wavelength which the said oscillations exhibit when propagating in the free space.

It was found that the transmission or production of oscillations of very high frequency with the aid of discharge tubes of the usual kind causes difliculties in practice when the wavelength of these oscillations was chosen of the order of magnitude of the size of the discharge tubes used. To avoid these difficulties, these tubes were given smaller and smaller dimensions, but as a result of this reduction, more particularly of thereduction of the cathode area, the power to be given oif by the discharge tube decreases to such extent that production or transmission of energy at the said very high frequencies no longer takes place. By shaping the electrode system of the discharge tube in the form of hollow waveguides whose dimensions satisfy the above-mentioned condition, it is now on principle possible to transmit or to produce oscillations of very high frequency with any desired power.

According to the invention, in the circuit utilizing the above-mentioned principle use is made of a magnetron tube for the production of electrical oscillations. Accordingly, the electrode system of the magnetron tube is shaped in the abovementioned manner in the form of a hollow waveguide preferably constituted by two concentric cylindrical envelopes the inner one serves as a cathode and the outer one as an anode.

The cathode may preferably have the shape of a perforated cylinder, preferably of gauze, which has one or more filaments arranged in it.

The hollow wave-guides may be open or closed at its end. For a proper operation it is desirable that the length of a closed hollow wave-guide measured in the direction of propagation of the oscillations should be smaller than half of the wavelength which the said oscillations exhibit when propagating inside this hollow wave-guide, whereas a hollow wave-guide which is not closed preferably has a length smaller than a quarter of the said wavelength.

In order that the invention may be more clearly understood and readily carried into effect it will be set out more fully by reference to the accompanying drawing.

Fig. 1 shows in perspective the electrode system for a magnetron tube in accordance with the invention,

Fig. 2 is a sectional view of said electrode system,

Fig. 3 is a view of the complete magnetron tube in conjunction with its operating circuits, and

Fig. 4 shows a modification of the wave guide structure in the magnetron of Fig. 3, the ends of the guide being enclosed.

Figure 1 shows the electrode system of a magnetron tube according to the invention. v This system consists of a hollow wave-guide constituted by two concentric cylindrical envelopes, an anode l and a cathode assembly 2. In the box.-

shaped space between the two envelopes at oneplace in the axial direction there is arranged a metal partition wall 3 constituted by a certain number of spokes connecting the inner and the outer envelopes.

According to the invention, the dimension 17 shown in Figure 1 has to satisfy a definite minimum condition. This dimension 7) is the dimension already mentioned before of the electrode system in the direction normal to the direction of propagation of the oscillations in the hollow wave-guide and normal to the direction of the electronic current in the tube, since the oscillations are propagating in the axial direction and the electronic current is in this connection assumed to be directed radially from cathode to anode. The dimension 2; which is in this case approximately equal to the average circumference, of the inner and the outer envelopes is consequently chosen larger than half of the wavelength (x) which the oscillations exhibit when propagating in the free space.

The length Z of the electrode system is determined by the wavelength (xx) which the oscillations exhibit when propagating inside the hollow wave-guide and also by the question as to whether the wave-guide is closed or not closed at its end.

The relation which exists between M and b is given by the following form:

From this form it follows that the value of M; is always higher than that of and that under definite conditions when is but slightly smaller than 21) this value itself may be very much higher than In order to obtain the highest possible energy, it is in the case of a closed hollow waveguide necessary for the length Z to be at most equal or smaller than whereas in the case of a non-closed hollow wave-guide it must be chosen Z Only in this case assist all points of the cathode in the same manner, hence in phase, in the production of the oscillations.

The described electrode system consequently has dimensions which need not be reduced with an increase in frequency of the oscillations to be produced, since the transverse dimension b is bound to a definite minimum and the dimension in the longitudinal direction may be given a value advantageous in practice by a correct choice of the relation x21), see the above-mentioned relation between ix, and D. From the roregoing ensues that, in spite of its comparatively large size, the discharge tube according to the invention can produce oscillations of very high frequency and, owing to this large size, more particularly owingto the large size of the electron-emitting surface, can on principle give oii any desired power.

For the oscillation of the magnetron tube it is not always necessary to provide the above-mentioned metal partition wall 3; the presence of this wall is, however; highly desirable since the manner of oscillation is determined by it. In fact, at the place of the partition wall is a current loop, so that in the vicinity of this wall the energy may be advantageously taken off by magnetic means. It is alternately possible to take off the high-frequency energy by electrical means, and this in points between which a voltage loop occurs; such points may exist, for example, on a quarter of the circumference, considered from the partition wall.

Figure 2 is a sectional view of the electrode system. The cathode assembly is constituted by an electrode 2' of gauze, which serves as a suction grid and has arranged in it a certain number of filaments 2". By reason of the direct connection between anode and suction grid 2, the latter is positively biased so that the effect of grid 2. is to increase the number of electrons drawn out of the space charge near filament 2". Some of these electrons are attracted to grid 2 but many of these pass through the openings in the grid into the wave guide space between grid 2 and anode I. extend in the axial direction but they may also be arranged in a different manner; thus, for example, it would be possible to mount a coiled filament at the inside of the electrode 2.

It is important to note that while member 2' is connected directly to anode I, it is not however to be regarded as part of the anode, for member 2 acts as a suction grid or space-charge grid which, in conjunction with filament 2", afiords a virtual cathode. The effect of the positively biased member 2' is to increase the number of electrons drawn out of the space charge near the filament. Some of these electrons are attracted to member 2 but many of these pass through its apertures into the wave guide space between member 2 of the cathode assembly and anode I. Thus the opposing walls of the wave guide are formed by the outersurface of member 2 and the inner surface of anode l.

The thickness of the hollow wave-guide is not In the figure the filaments fill bound to definite dimensions but preferably care is taken to see to it that the thickness is less than a quarter of the wavelength which the oscillations exhibit when propagating in the free space.

By a suit-able choice of the biassing voltages of the electrodes and of the strength of the magnetic field directed axially or substantially axially, it is possible to adjust the transit-time of the electrons over one turn in such manner that oscillations of the Ho-type are produced.

The electric field of a magnetron tube according to the invention shows some likeness with a field of the magnetron of the known kind in which the anode comprises two or more slits.

What we claim is:

. l. A magnetron oscillator adapted to generate oscillations of the Ho type at a predetermined wavelength, said oscillator comprising a virtual cathode system constituted by an electron emissive element surrounded by a tubular grid, a hollow cylindrical anode concentrically surrounding said grid, the opposing walls of said grid and said anode defining a waveguide, the mean value of the circumferences of said grid and anode exceeding half a wavelength of said oscillations in free space, and means connected to said anode for applying a positive voltage thereto relative to said emissive element.

2. A magnetron oscillator adapted to generate oscillations of the Ho-type at a predetermined wavelength, said oscillator comprising a virtual cathode system constituted by an emissive filament surrounded by a tubular suction grid, a hollow cylindrical anode concentrically surrounding said grid and defining an annular wave guide therewith, the mean value of the circumferences of said grid and anode exceeding half a wavelength of said oscillations in free space, and a conductive partition radially mounted in said wave guide and connecting said grid to said anode.

3. A magnetron oscillator adapted to generate oscillations of the ITO-type at a predetermined wavelength, said oscillator comprising a virtual cathode system constituted by an emissive element surrounded by a tubular foraminated memher, a tubular anode concentrically surrounding said member and defining an annular waveguide therewith, and a conductive partition radially mounted in said waveguide and connecting said member to said anode, the mean value of the circumferencesof said member and anode exceeding half a wavelength of said oscillations in free space.

4. A magnetron oscillator adapted to generate oscillations of the Ho-type at a predetermined wavelength, said oscillator comprising a virtual cathode system constituted by an emissive filament surrounded by a tubular foraminated member, a tubular anode concentrically surrounding said member and defining an annular waveguide therewith, and a conductive partition radially mounted in said waveguide and connecting said member to said anode, said partition being formed by radial spokes, the mean value of the circumferences of said member and anode exceeding half a wavelength of said oscillations in free space.

5. A magnetron oscillator adapted to generate oscillations of the Ho-type at a predetermined wavelength, said oscillator comprising a cathode assembly having a cylindrical configuration, a hollow cylindrical anode concentrically surrounding said cathode assembly and defining a waveguide therewith, the ends of said waveguide being enclosed, the mean value of the circumferences of said cathode assembly and anode exceeding half a wavelength of said oscillations propagated in free space, the length of said anode and said cathode assembly being less than half a wavelength of said oscillations in free space.

6. A magnetron oscillator adapted to generate oscillations of the Ho-type at a predetermined wavelength, said oscillator comprising a cathode assembly having a cylindrical configuration, a hollow cylindrical anode and concentrically surrounding said cathode assembly and defining an open-ended waveguide therewith, the mean value of the circumferences of said cathode assembly and anode exceeding half a wavelength of said oscillations in free space, the length of said anode and said assembly being less than a quarter of said free space wavelength.

7. A magnetron oscillator adapted to generate oscillations of the Ho-type at a predetermined wavelength comprising a cathode assembly having a cylindrical configuration, a hollow cylindrical anode concentrically surrounding said assembly, the opposing walls of said assembly and said anode constituting a wave guide, the mean KORNELIS SWIER KNOII. ALDER'I VAN DER. ZIEL.

REFERENCES CITED 10 The following references are of record in the file of this patent:

UNITED STATES PATENTS Number I Name Date 15 2,074,253 Dallenbach Mar. 16, 1937 2,108,900 Peterson Feb. 22, 1938 2,123,231 Dallenbach Aug. 30, 1938 2,248,712 Litton July 8, 1941 2,289,220 Smith July 7, 1942 20 2,289,846 Litton July 14, 1942 2,411,535 Fremlin Nov. 26, 1946 2,415,470 DeVore Feb. 11, 1947 FOREIGN PATENTS 5 Number Country Date 114,403 Australia Dec. 11, 1941 value of the circumferences of said opposing walls exceeding a half wavelength of said oscillations propagated in free space.

MAXIMILIAAN JULIUS OTTO STRUT'I. 

